State law. Where I live, the minimum is 6; a friend in Co says she can buy smaller numbers from her TSC. Now, our local feed store will let you get lesser numbers- the deal there is they "give away" 3 or 6 chicks, depending on the size of feed bag you get. Pretty smart, that. And then when they run out and you want more, they do a really big order after their chick days, which then allows people to get smaller numbers because they hatchery ships the large order to the feed store.
TSC, though, will let you split an order with someone else, if the manager approves it. I did that last year, but this year, I got 6. My complaint is that the only pullets they know the breed of they call "reds" which is supposedly all the hatchery labels them as being. I emailed TSC from the website because I wondered about store policy wrt never knowing what they were getting, and they said to call the day before and they should have the shipping slip to say what they would be getting the day before.
I wanted silkies, and had her pull out all the chicks with 5 feathered toes from the bantam bin, but they wouldn't let me get in there to do it. I think the laws are relatively good for protection of the onesies-twosies novelty gifts, but I do think if they know you and know you have chickens already, there should be some wiggle room.